In an effort to meet stringent federal emissions regulations, vehicle engines may be configured with an emission control system including various emission control devices, such as three-way catalysts, diesel oxidation catalysts, particulate filters, NOx catalysts, and hydrocarbon (HC) traps. The various emission control devices may be arranged in different configurations.
One example configuration is shown by Maaseidvaag et al. in U.S. Pat. No. 6,167,696. Therein, a three-way catalyst is included upstream of a HC trap, a NOx trap and an electrically heated catalyst. Another example configuration is shown by Yamato et al. in U.S. Pat. No. 7,181,903. Therein, a three-way catalyst is included downstream of a NOx trap and a plasma reactor containing a HC trap. Based on the specific configuration and order of the various emission control devices in the emission control system of '696 and '903, different control strategies (e.g., temperature control strategies) are used to coordinate their activities.
However, the inventors herein have identified potential issues with such systems. As one example, the various functions may compete and/or interfere with each other. For example, NO species may be oxidized to NO2 by a diesel oxidation catalyst upstream of a NOx catalyst after an engine cold start. However, the hydrocarbon and carbon monoxide oxidizing function of the diesel oxidation catalyst may interfere with the NO oxidation function of the catalyst. As a result, even with different configurations, emissions compliance may not be achieved. This issue may be exacerbated as emissions regulations get more stringent while combustion processes become more efficient with significantly lower exhaust temperatures. As another example, the different configurations and the lower exhaust temperatures may complicate emission control device temperature control strategies. As still another example, due to packaging volume constraints on the vehicle, the space available for the different configurations and functionalities of the emission control system may be limited. Overall, exhaust emissions may be degraded.
In one example, some of the above issues may be at least partially addressed by a layered emission control system coupled to a vehicle engine exhaust manifold. In one embodiment, the layered system comprises a first, upper layer including a first, oxidizing catalyst, a second, intermediate layer including a HC trap for trapping exhaust HCs, and a third, lower layer including a second, different oxidizing catalyst. The second layer may be positioned between the first and third layers, and all the layers may be layered on, and supported by a substrate support. In this way, various formulations may be layered on a substrate in a selected order to enable various emission control functions to be grouped within spatial constraints to provide synergistic benefits.
For example, an exhaust emission control system may include a layered emission control device upstream of one or more NOx catalysts and particulate filters. In one example, the layered emission control device may be a layered diesel oxidation catalyst system wherein a plurality of layers are layered on, and supported by a substrate support. A first, upper layer may include a first, oxidizing catalyst, such as a diesel oxidation catalyst (DOC), for oxidizing exhaust hydrocarbons (HCs) and generating an exotherm for downstream particulate filters or HC traps. For example, exhaust HCs may be oxidized to periodically generate an exotherm, such as to assist in regeneration of a downstream particulate filter. A second, intermediate layer may include a HC trap for trapping exhaust HCs. A third, lower layer may include a second, different oxidizing catalyst for oxidizing exhaust NO species to NO2 species. The exhaust NO2 species may then be trapped or converted in a downstream NOx trap or NOx reducing catalyst. By retaining the exhaust HCs in the second layer, the NO oxidation reaction of the third layer may be protected from HC interference. In addition to a specified functionality, each layer may also have a specified washcoat loading and/or precious metal loading suited to the specific functionality of the layer. The loadings may also be based on the vehicle application and the specific emission profile. For example, in vehicles operating on fuels or calibrations that have a higher HC content in the exhaust gas during cold start, the device may be configured such that the capacity of the second layer having the HC trap is increased. As another example, in vehicles operating with lean-burn engines that have a higher NOx content in the exhaust gas, the device may be configured such that the capacity of the third layer is increased to improve NO2 formation. In still further embodiments, one or more of the layers may be included within the substrate support, such as a highly porous substrate support, to reduce backpressure.
In this way, an emission control device may be configured with different formulations in the different layers to integrate various emission control functions and provide synergistic benefits. By reducing functional interference and improving functional synergy, the quality of exhaust emissions can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.